1. Field of the Invention
This invention relates to a composite sheet curable by a free radical reaction at relatively low temperatures within relatively short periods of time, and its object is to provide a composite sheet useful as an adhesive sheet or a surface layer-forming material of a sheet material such as a decorative sheet.
2. Description of the Prior Art
In recent years there has been a strong demand for increased simplification, or improving safety and sanitation in bonding operations in the adhesive field, and heat-curable adhesive sheets have widely superseded the old liquid adhesives. These heat-curable adhesive sheets, however, have the defect that high temperatures and long periods are required in order to obtain good adhesive strength.
For example, some epoxy resin adhesive sheets require heating at 150.degree. C for 2 to 3 hours, and the support is likely to be deteriorated as a result of heating at such high temperatures for such long times.
On the other hand, hot-melt adhesive sheets have been sold on the market as adhesive sheets that permit bonding within relatively short periods of time, but since their bond strength at high temperatures is poor, such adhesive sheets are not commercially significant.
Furthermore, in order to protect, decorate, or smooth the surface of a wooden sheet, metal sheet, plastic sheet or plywood, it has been the practice to heat bond to the surface thereof a surface layer-forming material prepared by impregnating a mixture of a thermosetting resin such as a phenol or melamine resin and a curing agent in a paper sheet, non-woven fabric or woven fabric. However, since this surface layer-forming material contains a thermosetting resin and a curing agent in the mixed state, its shelf life is poor.
The shelf life of such a surface layer-forming material could be prolonged by a proper choice of the curing agent, but the resulting surface layer-forming material has the defect that it requires relatively high temperatures and long periods of time to bond to the surface of a sheet material while heating, and, thus, the sheet material is undesirably deteriorated.